This invention relates to the bioleaching of zinc from an ore, such as a sulphidic ore, in a heap.
Conventional processes for the extraction of zinc mostly involve crushing, grinding and concentration of the ore and then roasting of the resulting concentrate followed by leaching, purification and electrowinning of the zinc. Metals such as copper, nickel and gold have been extracted using heap leaching. See for example U.S. Pat. Nos. 6,168,766; 6,110,253; 4,017,309; 5,196,052; and 4,721,526 where several copper and gold extraction processes are described. Heap leaching of nickel is described in Canadian Patent Application No. 2,155,050. Typically the heap leaching is applied to low grade ores, e.g.  less than 5 g/t for gold and  less than 1% for copper. Therefore, the part of the heap representing the metal being extracted is relatively small compared to the total amount of material in the heap. Nevertheless, the value of the metal being extracted renders the application of heap leaching for these metals economically feasible. Since small amounts of material are leached from such heaps, problems related to decrepitation, slumping and compaction are relatively minor concerns.
Zinc has a lower market value than copper or gold and particularly where the higher grade zinc ores are concerned, the tried and tested methods of concentrate roasting, leaching and electrowinning have been employed for the extraction of the ore. To the applicant""s knowledge there are no commercial heap bioleach processes in operation for the extraction of zinc. Therefore, despite the fact that heap leaching has changed the economics in so far as the recovery of gold and copper is concerned, this has not been applied to the recovery of zinc.
A reason for this may be the expectation that leaching of zinc presents problems unique to the heap leaching of zinc ores, such as the precipitation of iron oxides within the heap. Australian Patent Application No. 654322 states that a particular problem of treatment of transition ores in-situ or in heap is the tendency for iron present in the ore or in the treating liquor to precipitate as an insoluble precipitate leading to percolation problems.
While problems relating to decrepitation, slumping and compaction are relatively minor concerns in the heap leaching of copper and gold, these problems threaten to become major concerns with zinc ore leaching where considerable physical changes of the ore, especially with run-of-mine ores of good grade, can be encountered. These changes might be expected to result in percolation and irrigation problems, such as flooding, channelling and cold spots. In addition, permeability problems might be expected in view of the larger amounts of material that need to be leached from the heap in order to render the process economically feasible.
A bioleaching process for the recovery of zinc is described in WO01/18266 but this process is carried out in a reactor tank or vessel and employs oxygen enrichment in order to render the process feasible.
U.S. Pat. No. 6,096,113 describes a tank/heap biooxidation process for recovering a metal from a refractory sulphide ore by splitting the ore in two portions. The first portion is partially biodigested in a reactor to acclimatize the sulphide-digestion microorganism. The partially digested ore is then combined with the second portion. The resulting material is dewatered, biooxidized and subjected to lixiviation.
U.S. Pat. No. 5,429,659 describes a process for recovering precious or base metals from particulate refractory sulphide material comprising contacting the material with an aqueous solution containing a thermotolerant bacteria culture.
Hearne et al (1) propose a process for the recovery of zinc from its sulphide ores or concentrates by an entirely hydrometallurgical route. It consists of bacteria-assisted heap-leaching of sphalerite ore, or leaching zinc concentrate at elevated temperatures with ferric sulphate and re-oxidising the formed ferrous iron with the aid of bacteria in a ferric ion generator. The results of column leaching of different sized ores are reported. The economic feasibility of a moderate-scale heap leach operation is assessed but the authors conclude that for the purely hydrometallurgical recovery route for zinc, industrial acceptance is still some time away until the technology is fully developed and demonstrated on a large scale. The authors further conclude that heap leaching may be beneficial to recover zinc from marginal ore and foresee a process development stage in which a small heap leach/solvent extraction/electrowinning plant is incorporated as an xe2x80x9cadd onxe2x80x9d to another process, i.e. there is no teaching of heap leaching being operated as a stand alone process. The authors also state that zinc solvent extraction, crucial to both ore and concentrate leaching, is not yet satisfactorily solved.
Konishi et al (2) have reported on the kinetics of the bioleaching of ZnS concentrate by Thiobacillus ferrooxidans in a well-mixed batch reactor. Experimental studies were done at 30xc2x0 C. and pH 2.2 on adsorption of the bacteria to the mineral, ferric iron leaching and bacterial leaching. A mathematical model for bioleaching is presented for quantitatively examining the effects of certain operating variables with the object of selecting optimum bioleaching conditions for zinc concentrates.
Sandstrxc3x6m et al (3) have performed bioleaching with moderate thermophilic bacteria at 45xc2x0 C. and with extreme thermophilic archaea at 60xc2x0 C. on a complex zinc sulphide ore. The ore was fine grained and contained refractory gold as an additional value. The bioleaching was carried out in continuous stirred tank reactors and although the authors conclude that biooxidation of a complex zinc sulphide ore at 45xc2x0 C. and 65xc2x0 C. has proven to be a viable process, especially at the higher temperature, the ore must be finely ground (20 to 28 microns) in order to obtain high zinc recoveries at modest pyrite oxidation.
U.S. Pat. No. 4,401,531 describes a process for the recovery of zinc from secondary zinc raw materials by leaching followed by solvent extraction and electrowinning. However, again the leaching is carried out in stirred tank reactors and in this case no bio-leaching is involved.
Steemson et al (4) describe a process for zinc metal production from zinc concentrates by integrating zinc bioleaching with zinc solvent extraction and electrowinning. The bioleaching again was carried out in reactor tanks. The temperature was controlled at 40xc2x0 C. to 45xc2x0 C. In carrying out the process, the concentrate was slurried in water to produce a 6.5% w/w slurry which is then fed to the reactor tanks. See Australian Patent No. 673929 where the Steemson et al process is more fully described.
Krafft et al (5) report on the leaching of two Swedish zinc sulphide ores in columns. Five grain sizes ranging from 4 mm to 128 mm were used. The authors state that, despite intermittent sulphuric acid additions, pH values varied between 2.5 and 3.5 most of the time and it was impossible to avoid precipitation of iron compounds in the columns with the result that for the two smallest grain size fractions of the one ore, the column was clogged by precipitates of iron to such an extent that the leachate could not penetrate the ore mass.
Dutrizac (6) reports on ferric sulphate perolation leaching of a pyritic Znxe2x80x94Pbxe2x80x94Cu ore and states that zinc recovery from acidic iron-bearing solutions is difficult and that much work still needs to be done in this regard. It is further stated that the problem is especially severe for low zinc concentrations. Problems were also encountered when attempts were made to use higher iron concentrations since part of the iron precipitated.
In light of the above it can be seen that heap bioleaching of ore on its own, or in combination with solvent extraction and electrowinning, has not been established or proven as a viable process for the extraction of zinc on a commercial scale vis-à-vis the conventional processes involving roasting of the concentrate or the bioleaching of finely-ground zinc concentrates in reaction tanks where temperature, pH and bacteria concentration would be expected to be more even than with heap leaching.
It is an object of the present invention to provide a process for the extraction of zinc from an ore by means of heap bioleaching the ore and also to provide an integrated zinc extraction process which includes solvent extraction to produce a pure concentrated zinc solution for the production of zinc metal by electrowinning or for the production of zinc compounds. The invention is the basis of the Cominco HydroZinc(trademark) process.
According to the invention there is provided a method of extracting zinc from a sulphidic ore, comprising the steps of selecting a sulphidic ore having a maximum particle size of about 50 mm; forming the ore into a heap and bio-oxidizing the ore in the heap with acidophilic microorganisms by providing air to the bottom of the heap at a rate of at least 5 L/m2xc2x7min; and irrigating the top of the heap with an acidic solution containing up to about 30 g/L sulphuric acid at a rate of at least about 0.15 L/m2xc2x7min to produce a pregnant leach solution containing zinc in solution; and recovering zinc from the pregnant leach solution.
The units L/m2xc2x7min represent a volumetric flow rate (L/min) per unit area (m2) of the heap taken in plan view (i.e. in horizontal cross-section).
The acidic solution may contain from about 15 g/L to about 30 g/L sulphuric acid.
The pregnant leach solution may be subjected to solvent extraction to obtain a concentrated zinc solution and a raffinate. Preferably, zinc is only partially extracted during the solvent extraction. The amount of zinc extracted may be typically about 30% to 50% of the zinc in the pregnant leach solution. Thus, because less acid is generated in the solvent extraction, the solvent extraction may be carried out in stages without requiring neutralization between stages.
The concentrated zinc solution may be subjected to electrowinning to produce zinc metal. Alternatively, zinc may be recovered from the concentrated zinc solution in the form of a compound, such as zinc hydroxide, zinc sulphate, zinc oxide, zinc carbonate or zinc oxalate.
The ore may be derived from a sedimentary exhalative type deposit or a volcanogenic massive sulphide type deposit or, less preferably, from a carbonate replacement deposit.
The ore may also, for example, be any one of the following ores that still contains some sulphidic minerals: a complex ore, mixed ore, weathered ore, partially oxidized ore, oxidic ore and siliceous ore.
The ore may be a good grade run-of-mine ore containing at least 5% zinc or at least 10% and even more than 20% zinc but it may also contain a lesser amount, such as at least 3% zinc, if economically feasible. The process may also be applicable to low grade dump material of various grades.
The bio-oxidizing of the ore may be carried out at an average temperature of about 30xc2x0 C. to 85xc2x0 C., preferably about 35xc2x0 C. to 70xc2x0 C. The autogenous heating of the heap, from the heat of reaction, assists the leaching process.
The microorganisms may be indigenous to the ore or the ore may be inoculated with the microorganisms, e.g. by adding a culture or a solution containing the microorganisms, such as mine drainage solution with indigenous microorganisms, to the ore. Given the wide temperature range, the nature of the microorganisms can vary in the heap.
The process may further comprise the step of providing nutrient to the microorganisms. The nutrient may comprise nitrogen in the form of an ammonium salt and a source of potassium and phosphorous.
The ore may be a complex zinc-containing ore. It may be a zinc-copper ore in which case the pregnant leach solution contains both zinc and copper in solution. The zinc and copper may be recovered in separate solvent extraction circuits, e.g. copper may be extracted prior to removal of the zinc.
The ore may be derived from a sedimentary exhalative type deposit or a volcanogenic massive sulphide type deposit. The ore may contain zinc in the form of a zinc sulphide mineral, such as sphalerite, marmatite or wurtzite.
The heap may have a height of at least 2 meters, for practical reasons. The maximum height of the heap may also be limited due to certain physical constraints. It may have a height of from about 2 to 10 meters, preferably about 4 to 8 meters. Different configurations of heaps are possible from an engineering point of view.
The ore may be agglomerated before forming into the heap. The agglomeration may be effected with solutions at different acid strengths, including concentrated acid, depending on ore type and neutralization characteristics of the ore. The solution used for agglomeration may contain iron. The agglomeration may also be effected with pregnant leach solution, raffinate from the zinc solvent extraction or acid mine drainage.
The microorganisms may be mesophiles, thermophiles or extreme thermophiles, which are categories according to temperature ranges for growth. In this specification, mesophiles are those microorganisms that grow in the moderate temperature range up to about 45xc2x0 C. Thermophiles are heat-loving organisms having an optimum growth temperature in the range of 45xc2x0 C. to 60xc2x0 C. Extreme thermophiles have an optimum growth temperature above 60xc2x0 C. As mentioned above, the microorganisms may vary according to the temperatures in the heap.
These microorganisms may be selected from the following non-limiting examples of genus groups and species:
Acidithiobacillus spp. (Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Acidithiobacillus caldus), Leptospirillum ssp. (Leptospirillum ferrooxidans); Acidiphilium spp. (Acidiphilium cryptum); Ferromicrobium acidophilus; Ferroplasma acidiphilum; Sulfobacillus spp. (Sulfobacillus thermosulfidooxidans, Sulfobacillus acidophilus); Alicyclobacillus spp. (Alicyclobacillus acidocaldrius); Acidimicrobium ferrooxidans; Sulfolobus spp. (Sulfolobus metallicus); Acidianus spp. (Acidianus infernus); Metallosphaera spp. (Metallosphaera sedula); Thermoplasma spp. (Thermoplasma acidophilum).
According to another aspect of the invention the heap leaching may be effected with an acidic solution containing at least 30 g/L sulphuric acid.
Further according to the invention there is provided a method of extracting zinc from a sulphidic ore, comprising the steps of heap leaching the ore with an acidic leach solution in the presence of acidophilic microorganisms to produce a pregnant zinc solution; subjecting the pregnant zinc solution to zinc solvent extraction to obtain a manganese-free concentrated zinc solution and a raffinate; and subjecting the concentrated zinc solution to electrowinning to recover zinc therefrom, wherein the electrowinning is carried out in the absence of manganese in the concentrated zinc solution.
Also according to the invention there is provided a method of extracting zinc from a sulphidic ore also containing iron, comprising the steps of subjecting the ore to a heap leach with an acidic solution in the presence of acidophilic microorganisms to produce a pregnant leach solution containing zinc and iron; subjecting the leach solution to neutralization without the benefit of forced air flow to the leach solution, thereby maintaining the presence of the ferrous iron in the neutralized solution; subjecting the neutralized solution to zinc solvent extraction with an organic extractant to produce a loaded extractant and a raffinate containing ferrous iron in solution; stripping the loaded organic with an aqueous solution to produce a concentrated zinc solution; and recycling at least part of the raffinate to the heap leach. Furthermore, a portion of the pregnant leach solution, which contains iron, may be recycled to the heap so that the iron may assist in the leaching process.
The acidic solution preferably has sufficient acid content such that iron precipitation in the heap is avoided.
The pregnant leach solution may have a pHxe2x89xa64. It may have a pHxe2x89xa63.0 but preferably pHxe2x89xa62.5.
Further according to the invention there is provided a method of extracting zinc from a zinc solution, comprising the steps of subjecting the zinc solution to zinc solvent extraction to obtain a manganese free concentrated zinc solution and a raffinate; and subjecting the concentrated zinc solution to electrowinning to recover zinc therefrom, wherein the electrowinning is carried out in the absence of manganese in the concentrated zinc solution.
The zinc solution may be obtained by leaching a zinc ore or concentrate or by the leaching of an electric arc furnace dust or a recyclable zinc containing material.
Further objects and advantages of the invention will become apparent from the description of preferred embodiments of the invention below.